2. Why is irrigation important
• Not much new land waiting to be opened up for agriculture.
The main increase in food supplies has to come from making
better use of land already farmed
• More efficient crop production will help e.g. better varieties,
more use of fertilizer, mechanization, etc. but rainfall is still a
limiting factor for a very large proportion of the earth’s surface
and so irrigation is important in increasing production
• With irrigation, crop yields may be made more reliable, or two
crop a year may be possible instead of one
3. Advantages of Irrigation
1. To supply moisture which allows plant growth where
there was none before, or to get better growth, or to
extend the growing season
2. An insurance against drought
3. To allow the movement of plant nutrients. The
chemicals for plant growth are absorbed by the plant in
solutions eg surface application of nitrogenous fertilizer
will have no effect on a completely dry soil. There must
be water to dissolve it and take it down to the roots
4. To leach undesirable salts, that is to wash them out in
solution
5. To control the environment for growing plants. Eg
prevent frosts, to keep plant cool or to keep them moist
4. Limitations
1. Salinity
a. The water table is raised by irrigation, salts will build up into the root zone
b. Water containing dissolved salts for irrigation
In both cases, water is transpired by the plants and evaporated from the soil
surface but the salts remain behind so there is a progressive buildup over the
years.
2. Water logging
Too much water applied or inadequate drainage
Seepage from a canal
3. Erosion and sedimentation
Water flowing too fast in furrows or canals will cause soil erosion
Deposition of silt in canals is also undesirable
4. Damage to soil structure
Some kinds of sprinkler irrigation can damage the soil through erosive
application rates
5. Sprinkler Irrigation
Advantages:
1. Leveling is not required, so sprinkler can be used on hilly
or uneven land
2. Light, frequent watering are possible
3. Uniform distribution of water is possible
4. Amount of water can be easily controlled
5. No permanent field obstacles such as ditches
6. Small in-flow of water into the system can be used
7. The equipment can be mobile, and this is a vital
requirement for supplementary irrigation
8. The system is easy to set up
6. Disadvantages:
1. The capital cost of pumps, pipes and equipment is high
2. It needs a high power pump and a high flow rate of water
3. The size of droplet of water is not suitable for certain crops
esp. vegetables
4. The efficiency will be reduced by strong winds
5. Weed control will be affected
7. Types of Sprinklers and Sprays
1. Reaction rotation sprinkler
Most often used for lawns
The rotating head is driven around by the reaction of the
jets at the end of 2 or 4 arms
It is cheap but has little field application
2. Fixed head sprays
Used for lawns and orchard
The vertical jet strikes downward pointing cone and is
spread out in a flat cone spray
It is robust and no moving parts but gives a poor
distribution
8. 3. Perforated pipe
Light weight aluminum piping has many very small
holes drilled in the top of the pipe so that fine jets come
out at different angles and wet an area on either side
Droplet size is small but the rate of application is higher
4. Slow rotation sprinkler or Rain gun
Mostly used in field installations
Most sprinkler have either a single nozzle or two
opposing nozzles
2 Nozzles - One spray the inner area, the other called
the distance jet, extend the diameter of application
The nozzles are interchangeable and available in
different sizes to suit different pressures
9. Design of Sprinkler System
3 factors are interrelated and must be taken into account when
designing the system:
a. Soil factor
Moisture Holding Capacity (MHC, in/ft)
Coarse texture – low MHC
Fine texture – high MHC
Water infiltration rate (in/hr)
Coarse texture – high value
Fine texture – low value
b. Crop factor
Depth of root zone (inch)
c. Weather factor
Rate of Potential Evaporation (PET)
Hot weather – high PET
Cold weather – low PET
10. Drip Irrigation
is the frequent, slow application of water to
the soil for the purpose of sustaining plant
growth
this is done through mechanical devices called
emitter that are located at selected point along
water delivery lines
Most emitter are placed on the soil surface, but
they can be buried at shallow depth for
protection
Number of emitters depending on the size of
the plant, ranges from one to eight or more for
large trees
11. Advantages
1. Energy conservation
- Operating pressures are lower compared to sprinkler thus saving
pumping
energy
2. Water conservation
- the volume of water wetted by drip irrigation is usually less than
that
of other irrigation methods because between vegetative growth is
root irrigated
- important that approx. 33% of the soil in the wet zone is wetted
3. Additional vegetative growth
- when soil moisture is maintained at a high level with drip irrigation,
there is a noticeable increase in growth of plant
12. 4. Fertilizer efficiency
- areas of low rainfall, drip system are an effective means of getting
fertilizer into the soil moisture where it can be utilized by the
plants
- greater control over fertilizer placement and timing through the
system may lead to improved fertilizer efficiency
5. Low labour requirements
- the low application rates allow large areas to be irrigated at one
time for a given capacity of pump by opening and closing the
valves for each irrigation block
13. Limitation
Emitter clogging
- Because the water outlets or orifice in most
emitters are very small, they are easily become
plugged by particles of mineral and organic
matter
14. Filter
- one of the component of the whole system of
irrigation. It is used to prevent foreign particles such
as soil, sand and debris to move together with the
water in the pipe line and thereby causing clogging of
the emitters
15. Types of Filters
1. Screen or mesh (100 mesh filters)
2. Grooved disc
3. Sand media - for polluted pond or water
Filters are usually placed after the pump outlet and it can be
dismantled to clean the screen or grooved while sand media can be
cleaned by back washing
Emitters
- used to transmit and regulate the amount of water
needed by the plant through effective root zone
- discharged rate of emitter varies from 0.5, 1.0, 2.0 and
4.0 liter per hour
16. Water Pumps
Most commonly used is a centrifugal type pump,
operated by diesel engine or an electric motor
Size of the engine and pumps depends on the total
horsepower requirement of the system
If the total HP requirement is too high, divide the
system into 2 or more blocks of irrigation, and
irrigate one block at a time so that horsepower
requirement is reduced accordingly. This may
reduced the pump size and also the cost of having
the pump unit.
17. Piping System
1) Main, sub-main and lateral pipe lines
2) PVC is commonly used as the main and sub-main lines
3) The latest technology is using High Density Polyethylene (HDPE)
Tubing which can withstand high pressure, non-corrosive and durable
4) The sizes of main and sub-main depend on the discharge of water in the
system. The common size is between 20mm – 630mm (outside
diameter)
5) Low Density Polyethylene Tubing (LDPE) of diameter between 13mm
to 25mm is used for lateral lines. Recommended discharge rate for
13mm diameter LDPE is 1.0 gph
